The present invention pertains in general to battery packs and, more particularly, to a battery pack for obtaining information about the charge state of a battery and relaying it to an associated charging system.
With the advent of portable electronic systems for personal and business use, rechargeable batteries have seen an increased and widespread use. One problem that has risen with respect to rechargeable batteries has been the length of time that a battery can be used after it has been charged. Although manufacturers rate a battery for a given portable computer or battery operated device as to the number of hours that it will operate, the actual operating time is a function of a number of parameters. For example, personal computers provide a variable load which is a function of the peripheral devices that are activated, such as the screen, the hard disk, etc., such that the operating life of the battery will vary as a function of how often these peripheral devices are used. Power consumption by the peripherals has conventionally been reduced by utilizing increasingly sophisticated power management tools to minimize the amount of power that is drained from the battery, thus extending the operating time of the battery for a given charge. Another problem encountered with rechargeable batteries is partial charging. The manufacturers specifications are directed toward a relatively new battery with a xe2x80x9cfullxe2x80x9d charge. If, for some reason, the battery does not have a full charge, the user has no knowledge of bow much operating time he has on a particular battery. Since rechargeable batteries typically have a relatively flat voltage over their charge life, very little warning is typically available as to when the battery is nearing its end of discharge. Further, the condition of a battery, i.e., its age, etc., also effects the amount of charge that can be stored in a given battery. This could result in a fully charged battery driving the battery operated device for a shorter period of time than expected.
Battery-pack systems have been developed to provide information regarding the condition of the battery, the state of charge of the battery, etc., with these devices integrated into the battery-pack itself. Such a device is disclosed in U.S. Pat. No. 4,289,836, issued to Lemelson on Sep. 15, 1981. These battery-pack systems have been utilized with the battery to both condition the battery and provide some information as to the amount of charge that is in the battery, i.e., a measure of its capacity. These conditioning systems typically monitor and control the charging operation to determine when the voltage has reached a state that represents a full charge. Periodically, the battery is completely discharged for conditioning purposes. The capacity is learned by measuring the charge supplied to the battery from a condition where the battery is at its end of discharge voltage, and is charged to a full voltage. This charge monitoring circuitry is typically a device that measures current through a known resistive value and calculates charge therefrom.
One disadvantage to the present battery-pack systems is that in order to make some determination as to capacity, it is necessary for the charging operation to be an integral part of the capacity determining operation. However, systems having a totally self-contained charging unit that is separated from the battery have no way of storing capacity information with the battery after it is disconnected. As such, such systems do not provide battery capacity information.
Another disadvantage to battery pack systems is the use thereof with high current devices such as power tools. When a power tool is run under full load, charge removed from the battery increases to a very high level such that the voltage on the battery drops due to the voltage drop across the internal resistance of the battery. Since this voltage is utilized to determine various characteristics of the battery, determining the charge state thereof, etc., a high current condition can result in error. Further, a high current mode is typically followed by a zero current mode, wherein the battery again returns to its full voltage, even though the overall condition of the battery has deteriorated. Present systems do not account for a situation wherein current goes from a normal load level to a high load level.
One aspect of most battery monitoring systems is the requirement to monitor either a charging operation or a discharging operation. This is typically effected by placing a sense resistor in series with the current provided to the battery or taken from the battery. The voltage developed across the battery is then sensed and converted into a current value. This is either done through some type of analog-to-digital converter. However, due to the loss provided by the sense resistor, the sense resistor value is typically very small, such that the voltage across the resistor that is developed is also relatively small. To provide an adequate resolution, a very sensitive sensing device is required. Since the sensing device must deal with very small voltage changes, they typically will be subject to various offsets and the such. A differential system is typically utilized to remove external influences from the sensing operation. However, these differential circuits themselves have inherent imbalances which must be compensated for. In the past, these imbalances within the differential circuit have been accounted for by providing some type of offset cancellation operation. However, this provides an additional level of complexity.
The present invention disclosed and claimed herein comprises a battery monitoring circuit for monitoring current through a sense resistor disposed in series with one terminal of the battery by detecting the voltage across the sense resistor. The structure for sensing the voltage across the sense resistor utilizes a voltage/frequency converter that converts the voltage to a pulse stream. The rate of the pulse stream corresponds to the voltage across the sense resistor and, therefore, the current through the sense resistor. A circuit then determines the current through the sense resistor and the polarity thereof. The voltage/frequency converter has a differential structure associated therewith with first and second differential legs. The first differential leg has associated therewith passive switching elements and active elements and is operable to receive an input voltage and output a first differential signal. The second differential leg has associated therewith passive switching elements and active elements and is operable to receive the input voltage and output a second differential signal. The input voltage to the second differential leg is the voltage across the sense resistor. Multiplex circuitry is operable to periodically switch the passive switching elements and active elements in the first and second differential legs to the opposite of the first and second differential legs to provide balancing of inherent errors in the active elements and the passive switching elements.
In another aspect of the present invention, each of the first and second differential legs has associated therewith an input switched capacitor integrator. The switched capacitor integrator in the first differential leg has a first integrator feedback capacitor associated therewith and first switched control signals. Similarly, the switched capacitor integrated with the second differential leg has associated therewith a second integrator feedback capacitor and second switched control signals. A first switchable amplifier is provided that has associated therewith a switched capacitor input structure which, when the first switchable amplifier has the first or second integrator feedback capacitor connected between the input and output of the first switchable amplifier and the associated switched capacitor input structure controlled by the first or second switched control signals, it will form the respective one of the first or second switched capacitor integrators. Similarly, a second switchable amplifier is provided with an associated switched capacitor input structure. When the second switchable amplifier is configured with the first or second feedback capacitor connected across the amplifier and the associated switched capacitor input structure controlled by the respective first or second switched control signals, it will form the other of the respective first or second switched capacitive integrators. A multiplexer is provided that is operable to alternatively switch the first and second switchable amplifiers between the first and second differential legs.